1. Field of the Invention
The present invention relates to a voltage setting circuit used for setting an arbitrary voltage, adjusted by trimming a fuse, in a semiconductor integrated circuit.
2. Description of Related Art
FIG. 11 is a constitutional view of a conventional voltage setting circuit.
As shown in FIG. 11, a conventional voltage setting circuit arranged in a semiconductor integrated circuit is composed of a resistor 2 of a resistance valuer r.sub.2, a resistor 3 of a resistance value r.sub.3, a resistor 4 of a resistance value r.sub.4 and a resistor 5 of a resistance value r.sub.5 arranged in series in that order, a trimming fuse 7 arranged in parallel to the resistor 2, a trimming fuse 8 arranged in parallel to the resistor 5, and an output terminal 9 for outputting an output voltage V.sub.0 of a connection point between the resistors 3 and 4. A first reference voltage 1 of a voltage value Vref.sub.1 is applied to the resistor 2, a second first reference voltage 6 of a voltage value Vref.sub.2 is applied to the resistor 5. The trimming fuses 7 and 8 respectively have a very small resistance value as compared with the resistance values r.sub.2 to r.sub.5.
An operation of the voltage setting circuit is described. Because the resistance values of the trimming fuses 7 and 8 are very small, the output voltage value V.sub.0 of the output terminal 9 is expressed as follows.
V.sub.0 =(Vref.sub.1 -Vref.sub.2)/(r.sub.3 +r.sub.4)*r.sub.4 +Vref.sub.2 (1)
After a wafer process for manufacturing the voltage setting circuit is completed, the trimming fuse 7 or 8 is normally trimmed (or laser-cut) by using a laser beam to adjust the output voltage Vo of the output terminal 9.
In cases where the trimming fuse 7 is trimmed, the output voltage V.sub.0 is expressed as follows. EQU V.sub.0 =(Vref.sub.1 -Vref.sub.2)/(r.sub.2 +r.sub.3 +r.sub.4)*r.sub.4 +Vref.sub.2 (2)
Also, in cases where the trimming fuse 8 is trimmed, the output voltage value V.sub.0 is expressed as follows. EQU V.sub.0 =(Vref.sub.1 -Vref.sub.2)/(r.sub.3 +r.sub.4 +r.sub.5)*(r.sub.4 +r.sub.5)+Vref.sub.2 (3)
Therefore, the output voltage V.sub.0 at the output terminal 9 can be adjusted by appropriately trimming one of the trimming fuses 7 and 8. In this case, the number of choices in the output voltage V.sub.0 is 3 according to three equations (1), (2) and (3) by trimming one of the trimming fuses 7 and 8 in the voltage setting circuit using the four resistors 2 to 5.
In general, in cases where the number of choices in the output voltage is increased, even though the resistors of the voltage setting circuit are not uniformly formed in the wafer process, the voltage setting circuit in which the output voltage V.sub.0 is precisely set can be provided. Therefore, as shown in FIG. 12, in a second conventional voltage setting circuit, two resistors 11 and 12 of resistance values r.sub.11 and r.sub.12 are added, and two trimming fuses 13 and 14 having very small resistance values are added to be parallel to the resistors 11 and 12. In this voltage setting circuit, the number of choices is 5. Also, in cases where a plurality of trimming fuses can be trimmed, the number of choices is increased to 16. Therefore, because the number of choices in the output voltage V.sub.0 is increased, even though the resistors are not uniformly formed, a voltage setting circuit in which an output voltage V.sub.0 is precisely set can be provided.
However, in the above conventional voltage setting circuits, though the number of choices in the output voltage can be increased as the number of resistors is increased, an area occupied by the resistors is increased as the number of resistors is increased. Therefore, there is a drawback that a size of the semiconductor integrated circuit is increased.
Also, in cases where values of the output voltage V.sub.0 are set at equal intervals, it is convenient to adjust the output voltage V.sub.0 after the completion of the wafer process.
However, to set values of the output voltage V.sub.0 at equal intervals, it is required to precisely set the resistance values r.sub.2 to r.sub.5 of the resistors 2 to 5 to different values in a design step, so that there is another drawback that the resistance value design for the voltage setting circuit is troublesome.
Also, to obtain the output voltage V.sub.0 with a high precision, it is required to precisely set the resistance values of the resistors. Therefore, there is another drawback that it is difficult to obtain the output voltage V.sub.0 with a high precision after the completion of the wafer process.